Semiconductor laser devices are widely used in fiber optic communication systems. Distributed feedback (DFB) lasers have proven particularly successful in achieving narrow-linewidth, single-mode light for high-speed data transmission applications. The DFB device employs a Bragg grating structure for providing improved selectivity in the output light mode. The grating structure introduces a periodic modulation of the refractive index or gain within the laser cavity, thus facilitating single longitudinal mode oscillation within the cavity.
For low-error performance, conventional DFB lasers require optical isolation to prevent optical feedback, including external reflections, from coupling back into the laser chip. In a conventional DFB laser, the grating structure promotes the formation of two standing waves in the cavity (180° out of phase with each other), often referred to as the degenerate Bragg modes. In order to achieve single-mode, single-wavelength emission, the degeneracy must be “broken” so that only one of the Bragg modes will be pumped. Typically, the degeneracy is broken by reflections from the facets of the laser cavity, which strengthen one mode relative to the other, in essence “selecting” a single mode to lase. However, reflections from sources external to the chip may serve to break the degeneracy of the Bragg modes in an undesirable way, causing the laser output light to jump intermittently between multiple Bragg modes. Optical isolators are therefore required in order to maintain the single-mode, single-wavelength output light desired for optical communication. The need to shield the device from external optical feedback is particularly acute at high-data rates where dispersion effects may render long-span transmission impossible.
Typical optical isolators for use with DFB lasers include a pair of polarizers with an intervening Faraday rotator, or a polarization-maintaining pigtailed optical isolator, which is a passive device that allows light to travel through a fiber in one direction only. These isolators add significant cost to fiber optic communication systems, and complicate the construction of multifunction integrated devices, such as dense WDM systems employing compact multichannel, multiwavelength sources.